This invention relates to magnetic memory cell structures having a protective coating deposited over the magnetic surface and, more particularly, to memory cell structures of the type described wherein the protective coating is an electro-deposited layer of nonmagnetic nickel-phosphorous alloy.
Magnetic memory cells such as may be used with computer systems are often constructed of lengths of thin (e.g., 0.0025 inches) conductors having a magnetic surface. Information may be stored in such a memory cell by magnetic interaction set up at the intersection of the plated conductor with a second conductor sometimes referred to as a word strap. Coincident currents applied to the plated conductor and to the word strap results in the magnetic surface of the conductor in the area of the intersection being placed in a predetermined state of magnetization. To read information out of the memory cell, current is applied only to the word strap conductor and a readout current is induced on the plated memory cell conductor in accordance with the magnetization state of the magnetic coating.
Manufacture and assembly of plated wire memories of this type is an extremely delicate operation since the magnetic film provided on the wire is very thin (e.g. 4,000 to 6,000 A.U.) and soft and is highly susceptible to damage even when the most careful handling procedures are employed. Further, the magnetic coating, which may be a nickel-iron film deposited as set forth in U.S. Pat. No. 3,753,665 entitled "Magnetic Film Plated Wire", is also highly corrosive and is subject to rapid deterioration in a salt environment.
It has heretofore been the practice to make such plated wire more durable through the application of multiple layers of the magnetic material itself, or through application of an organic film or an electroless nickel alloy film over the basic magnetic layer. The use of multiple layers of the magnetic film is undesirable from the standpoint that it increases the cost of manufacture and, further, it does not resolve the corrosion problem. Organic protective layers involve the use of exotic and difficult to control deposition processes and further leave the wire with a nonconductive coating which prohibits use of on-line electrical testing techniques. Electroless films have the disadvantage of requiring high temperature application and also have undesirable magnetic properties which interfere with the basic operation of the memory cell itself.